A common type of layout for large multi-storied, multiple-unit dwellings (e.g., apartment houses}with internal corridors provides access to the dwelling units through corridors on each floor running off elevator and stair shafts. The corridors usually run along the center of the building lengthwise for access to side-by-side units on either side of the corridor. In this basic, widely used design all of the units, except for those at the ends of the building, have only one bounding wall available for openings for light and air. There are, of course, alternative forms of design to increase the number of units with two or more exterior aspects, such as direct access and hall access from more numerous elevators and stairways, but the alternatives involve increased costs per unit because of the greater burden of common space for elevators, stairways and hallways.
Another disadvantage of internal corridor access dwellings is a lack of privacy. With many units sharing a common corridor for access on each floor, the number of encounters with others in coming and going is greater, and such encounters are not always pleasant. Also, noise carrying along the corridors can be very bothersome to the occupants.